In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.

The human eye can function from very dark to very bright levels of light — its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly one thousand million apart. However, in any given moment of time, the eye can only sense a contrast ratio of one thousand. What enables the wider reach is that the eye adapts its definition of what is black. The light level that is interpreted as "black" can be shifted across six orders of magnitude—a factor of one million.

The merging of signals by virtue of the diffuse ganglion cells, as well as horizontal and amacrine cells, allow a cumulative effect. This means that area of stimulation varies inversely with the intensity, a strong stimulus over 100 rods or less is equivalent to one that is weak and over 1,000 rods. In sufficiently bright light, convergence is low, but during dark adaptation, convergence of rod signals boost. This is not due to structural changes, but by a possible shutdown of inhibition that stops convergence of messages in bright light. If only one eye is open, the closed eye must adapt separately upon reopening to match the already adapted eye. [1]

The fovea is blind to dim light (due to its cone-only array) and the rods are more sensitive, so a dim star on a moonless night must be viewed from the side, so it stimulates the rods. This is not due to pupil width since an artificial fixed-width pupil gives the same results.

The eye takes approximately 30 minutes to fully adapt from bright sunlight to complete darkness - one million times more sensitive than at full daylight. In this process, the eye's perception of color changes as well.

Cones and Rods in the eye are used during dark adaptation. Rods are more sensitive to light and so take longer to fully adapt to the change in light. This can take up to a few hours to completely regenerate. Cones take approximately 9 minutes to adapt to the dark.